A variety of different technologies, such as, for example, high speed, direct drive turbo-compressors, utilize high speed motors. Often, the operating speed of high speed motors, including asynchronous and synchronous motors, may be limited at least in part by the construction of the associated rotor. For example, due to the development of stresses in the magnet retention sleeve, typical permanent magnet motors may often be limited to achievable tip speeds of approximately 150 meters/second (m/s). Additionally, the laminated construction of asynchronous motors, such as, for example, induction motors, may also limit asynchronous motors to similar achievable tip speeds. Yet, to the detriment of the performance and reliability of the equipment that is being driven by the motor, lower tip speeds may impair the ability of the motor to obtain greater power density or more critical speed margins.
Prior attempts to overcome tip speed limitations have included the use of solid steel rotors. While such solid steel construction for the rotor may allow for higher achievable tip speeds, such as, for example, tip speeds in excess of 300 meters/second (m/s), and even tip speeds in excess of 500 meters/second (m/s), the electrical conductively of steel is typically too low for the associated machines to be designed with acceptable efficiency. Attempts to overcome conductivity deficiencies with solid steel rotors have included applying a copper coating to the solid steel rotor. Such construction utilizes the copper coating to carry an induced current at the surface of the rotor and to act as the rotor cage. Yet, with such construction, the peripheral speed of the rotor is limited to the coating strength and/or the strength of the bond between the copper coating and the steel rotor body. Further, the joining of dissimilar metals such as copper and steel, as well differences in their properties, such as, for example, melting temperatures, may limit the manner in which these two metals may be joined or bonded. For example, typically in rotor applications, the copper is applied to the steel rotor body via explosion welding, wherein the copper is able to come into close enough contact with the surface of the steel rotor body to form a weld. Yet, for at least certain applications, such bonding may be insufficient to allow the copper coated steel rotor to achieve the prerequisite degree of durability. For example, such bonding may be insufficient for turbo-compressor applications in which the motor is relatively frequently started and stopped and/or is exposed to relatively high temperatures.